Olefin Insertion and β-Elimination Reactions of Permethylniobocene Olefin Hydride and Permethylscandocene Alkyl Complexes

Abstract

Permethylniobocene styrene hydride complexes, Cp*2Nb(CH2=CHC6H4-m-X)H (Cp* = C5Me5, X = CH3, NMe2, CF3) have been prepared and their rates of olefin insertion measured by coalescence techniques. The rates compare favorably with those measured for the analogous para-substituted complexes, suggesting that electronic effects in the transition state are largely inductive in nature. The crystal structure of Cp*2Nb(CH2=CHC6H5)H was determined. The phenyl ring of the styrene is twisted out of resonance with the olefin to avoid unfavorable steric interactions with the bulky Cp* rings. The rates of ethylene insertion in the Sc-C bond for Cp*2ScR (R = CH3, CH2CH3, CH2CH2CH3 have been measured at low temperature by 13C NMR; the second order rate constants (M-1 sec-1, -80°C. The slow rate for the ScCH2CH3 complex is attributed to a ground state stabilization by a β-C-H interaction. The β-hydrogen elimination rates for a series of permethylscandocene alkyl complexes Cp*2ScCH2CH2R (R = H, CH3, CH2CH3, SiMe3, C6H5, C6H4-p-CH3, C6H4-p-CF3, and C6H4-p-NMe2) were measured by trapping kinetics. A transition state for the β-hydrogen elimination was proposed where there is a partial positive charge on the β-carbon and the hydrogen is transferred to the scandium center as H-. [Cp*2ScH] catalyses the ring opening of methylenecyclopropane and methylenecyclobutane. The ring opening (β-alkyl elimination) is reversible in the case of methylenecyclobutane. Addition of one equivalent of methylenecyclopentane to [Cp*2ScH] results in the formation of the scandium cyclopentylmethyl complex. This complex undergoes preferential β-alkyl (reversibly) over β-hydrogen elimination due to the unfavorable steric congestion encountered in the transition state of the latter. [Cp*2ScH] reacts rapidly at -80°C with ethyl vinyl ether and vinylfluoride to generate an equimolar mixture of the Cp*2ScX(X = OEt, F, respectively) and Cp*2ScCH2CH3. No intermediates are seen in these reactions and two mechanisms, one involving a β-X ethyl intermediate and one invoking direct σ-bond metathesis are proposed to account for the products. With vinyldiphenylphosphine, the initial product of insertion, Cp*2ScCH2CH2PPh2 is stable at low temperature; upon warming, this complex undergoes β-PPh2 elimination.